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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Pointers, constant values
Date: Fri, 16 Oct 2015 13:01:11 -0700
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Bartc <bc@freeuk.com> writes:
> On 16/10/2015 20:13, Keith Thompson wrote:
>> Bartc <bc@freeuk.com> writes:
>
>>>   int array_sum(int (*array)[], int n);
>>
>> Yes, but there's not much you can do with it, since you haven't
>> specified the length of the array it points to.  (I'm a little surprised
>> that that declaration is legal.)
>
> It's a pointer to an unbounded array. You can do anything you like with 
> it except anything involving its length. But int* doesn't have a length 
> either.

No, but int* specifies the size of the object it points to.  Given int
(*array)[], you don't even know the size of the element type.

> Actually, requiring a bound to be specified would severely restrict it 
> as you it would then only work for arrays of that length. Although you 
> might of course want that behaviour: if you implement a 3D point as 
> float (*)[3], you don't want anything passing other other kind of array.
>
>
>>> However this also now requires proper array dereferencing and
>>> indexing, not just applying an offset to an arbitrary pointer which
>>> can be to anything:
>>>
>>>   (*array)[i]
>>
>> That won't compile unless you have something between the [] in the
>> declaration.
>
> It works because I've tried it. (I wouldn't use this style of parameter 
> passing if writing actual C, it's too fiddly, but my generated code 
> always uses proper pointer-to-arrays.)

$ cat c.c
int array_sum(int (*array)[], int n) {
    array[0];
}
$ gcc -c c.c
c.c: In function 'array_sum':
c.c:2: error: invalid use of array with unspecified bounds
$ 

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"